Process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickle-based alloy surfaces

ABSTRACT

Aluminizing of nickel-base alloys and a process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys surfaces. Importantly, the process of producing the nickel aluminide (B2 NiAl phase) coatings on nickel-base alloys is directed to achieve high hardness of 800-1100 VHN and dimensional tolerance of 80±30 microns coating thickness. The process achieves desired nickel aluminide coatings with minimum environmental emission of toxic chemicals fumes and is also environment friendly. Obtaining the nickel aluminide coating is specifically possible without the need for masking of the unwanted regions of the components and more importantly can be attended both on flat and curved surfaces thereby favouring wide scale utilities of such aluminizing process, facilitate fabrication industries required to carry out surface modifications and engineering (protective coating) on nickel-base alloys desired for high service temperatures such as nuclear industries, aircraft and gas turbine industries.

FIELD OF THE INVENTION

The present invention relates to aluminizing of nickel-base alloys and, in particular, to a process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys surfaces. Importantly, the process of producing the nickel aluminide (B2 NiAl phase) coatings on nickel-base alloys is directed to achieve high hardness of 800-1100 VHN and dimensional tolerance of 80±30 microns coating thickness. Advantageously, the process of the invention achieves desired nickel aluminide coatings with minimum environmental emission of toxic chemicals fumes and is thus also environment friendly. Moreover, the nickel aluminide coating of the present invention is possible without the need for masking of the unwanted regions of the components and more importantly can be attended both on flat and curved surfaces thereby favouring wide scale utilities of such aluminizing process. The process of the invention would facilitate fabrication industries required to carry out surface modifications and engineering (protective coating) on nickel-base alloys desired for high temperatures such as nuclear industries, aircraft and gas turbine industries.

BACKGROUND ART

It is well known that nickel-base alloys are required to be provided surface modified and engineered for protective coatings to adopt such nickel-base alloys for high temperature application for variety of industrial and allied uses.

Aluminizing on nickel-base alloys is usually known to be carried out by processes such as pack cementations, slurry spraying, brushing, dipping and chemical vapour deposition to produce nickel aluminide (NiAl) layer over the substrate adapted for providing a hard surface for fretting wear and galling resistance (1-3).

It also known to carry out thermal spray processes alone for deposition of metals on worn out surfaces of steel components (4, 5). Also, it is well known to achieve pack diffusion and vapour phase diffusion coatings for protection against wear and fretting of fast reactor components (6-7). Aluminium paint-based processes for NiAl coating have also been used previously (8). While, the above existing state of the art prevails for the nickel aluminide coating on nickel base alloys there has been some inherent limitations/disadvantages of such hitherto known processes and devices.

Pack cementations is a process which has been widely used for such aluminizing since it is inexpensive and ideally suited for batch production of small components. In such pack cementation process, the components to be aluminized are usually treated at temperatures between 850° C. and 1050° C. in a pack consisting of an aluminium source such as Ni—Al, Ti—Al, or Cr—Al, an activator (halide) and some inert filler like alumina (1). It is however, experienced that such pack cementation technology involves handling of large quantities of alumina and metal powders, long furnace time cycles due to a large thermal inertia and inherently reduced throughput.

Vapour phase aluminizing (2) on the other hand largely eliminates the disadvantages of the pack cementation processes, but required specialized vacuum furnaces and fixtures. Moreover, both these processes involve exposure of operator to corrosive halide activators which is unsafe and undesired from the safety of the operator.

Apart from the above discussed limitations and complexities of the conventional methods of aluminizing on nickel-base alloys, it is also important that following such processes of aluminizing it has been always essential to mask the surfaces which were not required to be coated requiring special precautions and added complexities of such aluminizing processes.

OBJECTS OF THE INVENTION

It is thus the basic object of the present invention to provide for a process for NiAl aluminide coating of nickel-base alloys which would avoid the afore discussed limitations and complexities of the present processes/devices for carrying out surface modification and engineering (protective coating) of nickel-base alloys.

Another object of the present invention is directed to a process for NiAl aluminide coating of nickel-base alloys which would on one hand be cost effective and at the same time on the other hand would involve safe technology without the problems/disadvantages of handling of large quantity of alumina and metal powder, long furnace time cycle due to large thermal inertia and inherent reduced throughput.

Further object of the present invention is directed to a simple process for NiAl aluminide coating of nickel-base alloys which would also not require any specialized vacuum furnaces and fixtures and thus can be readily applied and used for producing aluminide nickel-based alloys.

A further object of the present invention is directed to an aluminizing process which would favour achieving NiAl aluminide coating on nickel-base alloys and which would be suitable for mass production of large sized components to a tolerance of 80±30 microns with hardness range of 800-1100 VHN with reduced cycle time than the conventional pack cementation and vapour phase diffusion coating.

Yet another object of the present invention is directed to favour production of selectively body centered cubic (B2) nickel aluminide (NiAl) coated nickel-base alloys of desired thickness without the complexities of masking of surfaces not to be coated thereby facilitating controlled and simple production of such nickel aluminide coated nickel-base alloys.

A further object of the present invention is to produce ordered body centered cubic (B2) nickel aluminide (NiAl) coated nickel-base alloys which can be advantageously used to form the coating only on the desired surfaces without any special steps to masking the areas where aluminizing is not desired.

Yet another object of the present invention is directed to, produce ordered body centered cubic (B2) nickel aluminide (NiAl) coated nickel-base alloys with higher productivity, environmental cleanliness and suitability for mass production of large sized components.

Yet another object of the present invention is directed to a process which would achieve NiAl aluminide coating on nickel-base alloys involving flat and/or corrugated surfaces to thereby favour production of variety of NiAl aluminide coated nickel-base alloys.

A further object of the present invention is directed to produce surface protective coating for high temperature application on nickel-base alloys with selectively desired coating thickness, micro structure and hardness which can be consistently reproduced.

Yet further object of the present invention is to provide for a simple yet effective process for producing surface protective coating for high temperature application on nickel-base alloys which would favour overall reduction of time and cost for aluminizing vis-à-vis the conventional processes of aluminizing of such nickel-base alloys presently in use.

SUMMARY OF THE INVENTION

Thus according to the basic aspect of the present invention there is provided a process for producing nickel aluminide (NiAl) coating preferably body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloy surfaces comprising:

thermal spraying of the required surfaces with commercial pure aluminium wires to desired thickness; subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment such that the aluminium sprayed reacts with the nickel-base alloy substrate and form the desired aluminum coating on the entire nickel-base alloy substrate or selective required surface thereof to be coated and with controlled thickness.

In accordance with a preferred aspect of the present invention there is provided a process for producing nickel aluminide (NiAl) coating body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloy surfaces comprising:

activating the surfaces to be coated by grit blasting preferably following dimensional check and degreasing; thermal spraying of the required surfaces with commercial pure aluminium to desired specific thickness in the range of 100 to 200 microns; subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment in vacuum atmosphere such that the aluminium sprayed reacts with the nickel-base alloy substrate and form the desired aluminum coating on the entire nickel-base alloy substrate or selective required surface thereof to be coated and with controlled thickness; and subjecting the coated surface to surface cleaning.

Importantly, in the above process for producing body centered cubic (B2) nickel aluminide (NiAl) coating the said surface activation by grit blasting is carried out using alumina grit of specific size and distribution preferably in the range of 30-120 mesh size at an air pressure of 5.5+/−0.5 kg/cm².

In accordance with another aspect of the invention, the thermal spray parameters comprise (i) aluminum wire diameter in the range of 3.0 to 3.2 mm at a Wire Feed Rate in the range of 2.0-2.5 mm/sec; (ii) Pneumatic pressure in the range of 5.5+/−0.5 kg/cm²; (iii) Oxygen Pressure in the range of 2.5+/−0.3 kg/cm²; and Acetylene pressure in the range of 1.0+/−0.2 kg/cm².

The said diffusion heat treatment in said vacuum atmosphere is carried out preferably (10⁻⁵) level at temperature in the range of 950-1100° C. for a duration of 1.0 to 1.5 hours.

Preferably, in the above process for producing body centered cubic (B2) nickel aluminide (NiAl) coating the said surface cleaning of the coated surface comprise removing adherent oxide scale using pickling solution following the steps of:

-   (i) providing the pickling solution preferably comprising (a)     concentrated nitric acid (specific gravity: 1.41 g/cm³) in amounts     of 16% (by volume); (b) Hydrofluoric acid (specific gravity 1.61     g/cm³) in amounts of 3.5% (by volume); and (c) Water in amounts of     80.5% (by volume); -   (ii) soaking the diffusion treated strips in the solution for a     period of about 30 minutes; followed by -   (iii) further cleaning the strips involving emery papers of 400 grit     size.

Following the above disclosed process of the invention it is possible to carry out the NiAl aluminide coating on nickel-base alloys involving selectively flat and/or corrugated surfaces. Importantly, the said NiAl aluminide coating is carried out on nickel-base alloys with minimum 40 weight % nickel content on both flat and corrugated surfaces.

The process is adapted to produce surface protective coatings selectively free of any masking of the uncoated regions for desired high temperature applications on nickel-base alloys with minimum 40 weight % nickel content and having hardness of 800-1100 VHN and a coating thickness of 80+/−30 micron thickness.

Importantly, in the above process the surface activity, spraying procedures and diffusion parameters are critical in optimization of the coating thickness and hardness. Surface activity is achieved by selection of suitable grade grit size (30-120 mesh size) during grit blasting. Thermal spraying of aluminium is carried out using 3.2 mm diameter aluminium wires and by maintaining a torch-work distance of 200-300 mm and wire feed rate of 2.0-2.5 mm/sec. Thickness of sprayed layer is monitored on 100% of the qualification strips and on a certain percentage of production strips at random to ensure sprayed layer thickness of 100-200 microns. The optimized diffusion treatment is carried out at 950-1100° C. for 1.0-1.5 h.

It is thus possible by way of the present invention discussed above to produce ordered body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys involving a selective thermal spray-diffusion process. Importantly, the process of the invention achieves selective advantages in producing such coated nickel-base alloys by involving the thermal spraying which is a process in which molten or semi-molten particles are applied by impact on to the surface at uniform rate using mechanise or manually operated spray gun.

The selective diffusion treatment after thermal spray in vacuum atmosphere is involved whereby the aluminium melts and reacts with the nickel/iron base alloys to thereby form the desired B2 phase.

Importantly, the major advantage in the above process of the invention involving the combination of thermal spraying and diffusion treatment is that the same avoids the complexities of masking of uncoated surfaces and yet can favour achieving the coating only on the desired surfaces of the nickel-base alloys. This is an important advantage in the present process of producing the nickel aluminide coated nickel-base alloys of the invention since the conventional coating process of such alloys such as the conventional pack cementation process essentially required coating the entire strip unless special steps are taken to mask the area where aluminizing is not desired.

The process being simple to operate and apply favours for higher productivity and is importantly environment friendly and safe from the operators point of view since the same does not require the operator to be exposed to corrosive environment such as corrosive halide activators involved in the conventional pack cementation and vapour phase aluminizing processes.

The details of the invention, its object and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustration of the process of the invention as per the following accompanying FIGURE and examples:—

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURE

FIG. 1: is a schematic illustration of the stages involved in the aluminizing of Ni-base alloys in accordance with the present invention involving thermal spray and diffusion treatment.

As shown in said FIGURE the basic steps involved in such process of aluminizing comprises of carrying out sequentially (a) dimensional checks, (b) degreasing, (c) grit blasting, (d) thermal spraying, (e) diffusion treatment (f) surface finish and (g) dimensional check. Such a process of aluminizing involving the thermal spray and diffusion treatment of the invention is illustrated further by way of the following example.

EXAMPLE

An Exemplary process of aluminizing of nickel-base alloys following the process of the invention involving the thermal spraying and diffusion treatment was carried out as detailed hereunder:

-   -   i) The nickel-base alloys surface to be coated was first         activated, after dimensional check and degreasing, by grit         blasting using alumina grit of specific size and size         distribution i.e., in the range of 30-120 mesh size at a air         pressure of 5.5±0.5 kg/cm²;     -   ii) thereafter thermal spraying of the required surfaces using         commercially pure aluminium (dia 3.2 mm) to specific thickness         in the range 100 to 200 microns was carried out using the         following thermal spray parameters         -   thermal spray parameters:         -   a) wire feed rate: 2.0-2.5 mm/sec.;         -   b) pneumatic pressure: 5.5±0.5 kg/cm²;         -   c) oxygen pressure: 2.5±0.3 kg/cm²; and         -   d) acetylene pressure: 1.0±0.2 kg/cm².     -   iii) subsequently diffusion heat treatment was carried out in         vacuum atmosphere (10⁻⁵) level at specified temperature in the         range 950-1100° C. for a duration of 1.0-1.5 h to enable the         aluminium sprayed to react with the nickel—iron—chromium alloys         substrate and form the aluminide coating;     -   iv) subsequently surface cleaning procedure was carried out to         remove adherent oxide scale using a pickling solution as per the         following details:         -   a) concentrated nitric acid (specific gravity: 1.41 g/cm³):             16% (by volume);         -   b) hydrofluoric acid (specific gravity 1.61 g/cm³): 3.5% (by             volume); and         -   c) water: 80.5% (by volume).

The pickling solution obtained as above was used to soak the diffusion treated strips for 30 minutes followed by hand cleaning of the strips such as by using 400 grit size emery papers.

By way of the above disclosed process of the invention it was possible to produce the ordered body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys involving the simple yet effective thermal spray-diffusion process. Advantageously, the present aluminizing process can achieve NiAl aluminide coating on nickel-base alloys with minimum 40 wt % nickel content on both flat and corrugated surfaces. Moreover, the processes found suitable for mass production of large sized components to a tolerance of 80±30 microns of NiAl aluminide coating with hardness range of 800-1100 VHN with reduced cycle time as compared to the conventional processes such as the pack cementation and vapour phase diffusion coating. Moreover, the process of the invention involving the selective combination of thermal spray and diffusion heat treatment would favour coating on selected areas of the components both on flat and curved surfaces without the need for masking and its related complexities. Thus, the process offers overall reduction of time and cost of aluminizing which is found to be four time faster than the conventional processes of aluminizing nickel-base alloys.

The present process would thus enable producing surface protective coatings for high temperature applications on nickel-base alloys with desired coating thickness, microstructure and hardness which importantly can be consistently reproduced. Moreover, the process would favour the desired surface areas alone to be aluminized without the need for complex masking of uncoated areas. The process offers overall reduction of time and cost for aluminizing. The duration of aluminizing for other conventional processes is four times higher than the present invention and the process would therefore favour much simpler and faster generation of NiAl coated Ni-base alloys both on flat and/or corrugated surfaces apart from being environment friendly and safe to carry out. 

1-10. (canceled)
 11. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness of 50 to 110 microns with high hardness in the range of 800-1100 VHN on nickel-base alloy surfaces with minimum 40 weight % nickel content comprising: activating the surfaces to be coated by grit blasting preferably following dimensional check and degreasing; thermal spraying of the required surfaces with commercial pure aluminium to desired specific thickness in the range of 100 to 200 microns preferably 100-120 microns; and subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment in vacuum atmosphere at a temperature in the range of 950° C.-1100° C. for a duration of 1.0 to 1.5 hours such that the aluminium sprayed reacts with the nickel-base alloy substrate to form the desired nickel aluminide coating of said controlled thickness and hardness on said nickel-base alloy substrate and subjecting the coated surface to surface cleaning.
 12. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to claim 11 wherein the said surface activation by grit blasting is carried out using alumina grit of specific size and distribution preferably in the range of 30-120 mesh size at an air pressure of 5.5+/−0.5 kg/cm² and more preferably with 50-80 mesh size at an air pressure of 5.0-5.1 kg/cm².
 13. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to claim 11 wherein said thermal spray parameters comprise (i) aluminium wire diameter in the range of 3.0 to 3.2 mm at a Wire Feed Rate in the range of 2.0-2.5 mm/sec; (ii) Pneumatic pressure in the range of 5.5+/−0.5 kg/cm²; (iii) Oxygen Pressure in the range of 2.5+/−0.3 kg/cm²; and Acetylene pressure in the range of 1.0+/−0.2 kg/cm².
 14. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to claim 11 wherein said diffusion heat treatment in said vacuum atmosphere is carried out at (10⁻⁵) millibar pressure preferably at temperature in the range of 1010±10° C. with an aluminium spray thickness of 100-120 microns with the resultant coating thickness and hardness being 80-90 microns and 980-1030 VHN respectively.
 15. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to claim 11 wherein the said surface cleaning of the coated surface comprise removing adherent oxide scale using pickling solution following the steps of: (i) providing the pickling solution preferably comprising (a) concentrated nitric acid (specific gravity: 1.41 g/cm³) in amounts of 16% (by volume); (b) Hydrofluoric acid (specific gravity 1.61 g/cm³) in amounts of 3.5% (by volume); and (c) Water in amounts of 80.5% (by volume); (ii) soaking the diffusion treated strips in the solution for a period of about 30 minutes; followed by (iii) further cleaning the strips involving emery paper.
 16. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to claim 11 wherein the NiAl aluminide coating is carried out on nickel-base alloys involving selectively flat and/or corrugated surfaces.
 17. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to claim 11 adapted to produce surface protective coatings such as to improve wear resistance on nickel-base alloys with minimum 40 weight % nickel content for high temperature applications.
 18. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloy with minimum 40 weight % nickel content surfaces substantially as herein described and illustrated with reference to the accompanying FIGURES. 